The present invention relates to a method and apparatus for studying the sub-soil and its electro-magnetic properties. It is especially adapted to be used in mining, oil, geothermal and hydrology prospecting, and whenever there is need accurately to locate an anomaly (i.e. discontinuity) in the electric conductivity of the sub-soil.
The magnetic and electric fields in the ground undergo continuously changing fluctuations which have their origin in ionospheric disturbances which themselves result from solar activity. Since their origin is very remote, if the sub-soil were homogeneous, such fluctuations should be quite uniform over distances of the order of tens of kilometers and more. On the basis of these observations, a geophysical method was developed several decades ago by Conrad Schlumberger and his co-workers. According to that method, known as the telluric method, simultaneous recordings of natural underground electric currents, called telluric currents, at two stations separated by a given distance are compared. When the recordings cannot be superimposed one can conclude that the composition of the sub-soil between the two stations differs. With a few calculations and corrections, an analysis of the variations of the recordings from the various stations will give some indication regarding the structure of the sub-soil.
This method, however, produces results which are difficult to interpret and are much more qualitative than quantitative in character. Moreover, the necessity for at least two simultaneous recordings at different stations is cumbersome since in order to obtain an acceptable accuracy, it is necessary to deploy at each station an insulated wire several hundreds of meters in length. For these reasons the above discussed method was superseded by the so-called magneto-telluric method developed by Louis Cagniard around 1950 (see French Pat. No. 1,025,683). In that method an electric field at a point of the ground surface is compared with the magnetic field with which it is connected by the general equations of Maxwell which relate the character of an electric field with that of an associated magnetic field.
The need to make two recordings of the electric field at two different points is thus avoided by the magneto-telluric method. Moreover, the ratio between the magnetic and the electric field amplitudes can easily be used to supply indications regarding the structure of the sub-soil. In this regard, use is made of the so-called "skin effect" phenomenon according to which as the frequency of the naturally induced electro-magnetic wave decreases the depth of penetration of the wave increases.
Until recently the electro-telluric or magneto-telluric recordings were interpreted by using Fourier transformations, that is to say by considering the variations of the magnetic and electric fields as periodic phenomena. In the course of the past few years, there has come about an improvement of the magneto-telluric method in which the field variations are considered as exponential phenomena rather than as periodic phenomena (see Geothermics, Vol. 2, No. 2, June 1973). Given a modification of the measuring apparatus, it has thus been possible to obtain an increased accuracy because there have been taken into account in a practical manner the variations in the vertical component of the magnetic field.
Thus, one can see that the various prospection methods just described have been successively improved. They are now well suited for use in cases in which the structures of the sub-soil and in particular of mineral deposits are horizontal or slanted, with regular or variable slopes (cylindrical or pseudo-cylindrical structures), provided that the conductivity contrasts or differences are sufficiently pronounced.
The above-discussed methods, however, are not well suited for sub-soil structures or deposits having a vertical or sub-vertical slope as sometimes found in the course of mining or geothermal explorations in which the deposits are generally vertically oriented and have limited horizontal surfaces. This limitation results from the poor localization characteristics of the existing methods involving long lengths of electrical lines used. In effect, the measurement accuracy of the electric field is a function of the length of the electric wire extended over the ground for registering the voltage differential potential between its ends.
It would, of course, be possible to use very short electrical lines and to provide for a suitable amplification. However, parasitic (stray current) phenomena such as spontaneous polarizations, variations in the nature of the ground in the immediate neighborhood of the electrodes, etc. tend to frustrate such a method. In practice, the electrical lines usually have a length in the order of from one hundred to several hundred meters. Moreover, it is difficult to locate in the sub-soil anomalies or deposits having dimensions in the order of a few meters, or tens of meters with the necessary accuracy for detecting faults which might contain minerals, fractures or breaking zones in a calcareous reservoir, the edge of a small fossil reef more or less invaded with hot or salted water, etc.
It should also be noted that high population densities can also adversely affect the electric lines due to industrial or natural stray currents emanating from sources in the vicinity of the lines.
A primary object of the present invention is to improve prospecting methods to eliminate the above-mentioned drawbacks and limitations, especially for the making of maps, profiles or apparent conductivity electrical tables.
In order to better indicate the scope of the invention, it is suitable to recall that, in the case of terrains with a tabular distribution, it is very well-known both theoretically and experimentally that the respective vertical components of the electric and magnetic fields are null.
This is no longer the case when in the midst of the sub-soil there occurs a sudden discontinuity of the type already indicated above, such as a fault or interruption of any type, a change of features, the sudden appearance of a reservoir presenting geothermal interest, etc.
It can even be said that the presence of such a discontinuity in a given sub-soil can be detected through the existence of a natural vertical electric component and a natural vertical magnetic component the variations of which are located in a given part of the electro-magnetic spectrum as a result of the skin effect. These variations will depend on the arrangement, the depth and the resistivity distribution which characterize said discontinuity, as well as on the surrounding milieu.
For the purpose of increasing the accuracy of locating mineral deposits or other sub-soil discontinuities on a commercial scale the present invention makes simultaneous measurements of the variations in the vertical magnetic component at a given point and as a function of time. It also measures the variations of a quantity "U" directly related both to the variations of the natural vertical electric component as a function of time, and to the depth-resistivity distribution in the underlying ground. A suitable analysis of the signals thus recorded makes it possible to obtain physical non-variables which are representative of the subjacent ground at the point of measurement.
For the sake of clarity, it is appropriate to recall one of the fundamental relationships in electro-magnetism. There are first arbitrarily defined at the surface of the ground two axes of coordinates Ox and Oy, which cross at point O and which define the horizontal surface at the point under consideration. Axis Oz defines the descending vertical. H.sub.x is the natural magnetic component along axis O.sub.x ; H.sub.y is the natural magnetic component along axis O.sub.y. It is known that: EQU 4 .pi. U = .delta.H.sub.y /.delta.x - .delta.H.sub.x /.delta.y, with U = .sigma.E.sub.z
E.sub.z represents the natural vertical electric component and .sigma. represents the electric conductivity of the medium at the surface at which point O is placed.
Moreover, the natural vertical electric component E.sub.z is related for a given component of the electro-magnetic spectrum, to the various components H.sub.x, H.sub.y, H.sub.z through the expression: EQU E.sub.z = .lambda.H.sub.x + .mu.H.sub.y + .nu.H.sub.z
in which .lambda., .mu. and .nu. are constants.
One then proceeds at a given point and as a function of time, to simultaneously measure component H.sub.z and the two components H.sub.x and H.sub.y, as well as to measure the variations of H.sub.x when moving a small distance L along axis O.sub.y and the variations of H.sub.y when moving the same distance along axis O.sub.x. It is then possible, through a suitable analysis of the signals thus recorded, to obtain physical non-variables which are useful to the prospector, as will be explained below.
A measuring device constructed according to the present invention essentially includes a set of five magnetic sensors or detectors, respectively characterized by response curves which are identical among themselves or which present known relationships, each one of these sensors being "directional", that is to say each one presents a favored direction in which it is most sensitive to the variations of the magnetic field as a function of time.
The five sensors are placed in the following manner: three sensors C.sub.x, C.sub.y, C.sub.z are oriented in three different respective directions X, Y and Z, direction Z not being parallel with the surface of the ground; a fourth sensor is placed outside the plane defined by sensor C.sub.x and directions X and Z, its orientation being different from those contained in plane YZ; a fifth sensor is placed outside the plane defined by sensor C.sub.Y and directions Y and Z, its orientation being different from those contained in plane XZ.
According to a preferred arrangement, direction Z is approximately vertical; directions X and Y are approximately horizontal and perpendicular to each other; and the orientation of the fourth and fifth sensors are parallel to X and to Y, respectively. This ensures the most precise data and the simplest calculations.
The invention further includes means for simultaneously recording, as a function of time, the variations of the components of the magnetic field which have been detected by the five sensors.